The present invention relates to a method for orientating an optical polymer film by oblique stretching, a method for manufacturing a polarizing film with high yield, a polarizing film, polalizer and birefringencial film using the optical polymer film obtained by the method, and a liquid crystal display device using the polarizer.
Demand for polarizers has surged with the diffusion of liquid crystal display devices (hereinafter referred to as LCDs). In a polarizer, a protective film or protective films are generally laminated on one or both sides of a polarizing layer having polarizing ability through an adhesive layer.
As a raw material for the polarizing layers, polyvinyl alcohol (hereinafter referred to as PVA) has been mainly used. PVA films are dyed with iodine or dichromatic dyes after uniaxial stretching, or stretched after dying, and further crosslinked with boron compounds, thereby forming polarizing films for the polarizing layers.
As the protective films, cellulose triacetate has been mainly used, because it is optically transparent and has low birefringence. Usually, the films are longitudinally uniaxially stretched, so that absorption axes of the polarizing films are longitudinally roughly parallel.
In the conventional LCDs, transmission axes of the polarizers are arranged, inclined at 45xc2x0 to the longitudinal or lateral direction of images. Accordingly, in the stamping process of the polarizers produced in the roll form, stamping has been carried out in a direction inclined at 45xc2x0 to the longitudinal direction of the rolls.
However, stamping in the direction at 45xc2x0 generates unusable portions in the vicinity of ends of the rolls. In particular, large-sized polarizers have the problem of a low yield. Further, for the polarizers after lamination, it is difficult to recycle materials thereof, which introduces the problem of increased waste.
Birefringencial films are used by adhering them to the polarizers or the like forming the LCDs, for optical compensation such as coloring prevention and enlargement of the angle of visibility, and it is needed to set orientation axes of the birefringencial films at various angles to the transmission axes of the polarizers. Conventionally, a system of cutting longitudinally or laterally uniaxially stretched films by stamping out film thereof in such a manner that orientation axes thereof give specified angles inclined to the edge thereof, which has raised the problem of a low yield, similarly to the polarizers.
For solving this problem, some methods are proposed in which the orientation axes of polymer films are inclined at desired angles to film transferring directions. JP-A-2000-9912 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposes that a plastic film is longitudinally or laterally stretched at speeds different from each other on the right and left to the stretching direction, while uniaxially stretching the film in a lateral or longitudinal direction different from the above-mentioned stretching direction, thereby inclining the orientation axis thereof to the above-mentioned uniaxial stretching direction However, according to this method, when a tenter system is used, for example, it is necessary to give the difference in transferring speed between the right and left to the stretching direction. As a result, crease due to uneven stretching stress, wrinkles and local unevenness of film thickness caused thereby are developed, resulting in difficulty in obtaining a desired inclined angle (45xc2x0 in the polarizer). When the difference in speed between the right and left is intended to be decreased, the stretching stage must be lengthened, which causes installation cost to be greatly increased.
Further, JP-A-3-182701 proposes a method for producing a film having a stretching axis of any angle xcex8 to a running direction of the film, which comprises holding the continuous film at both of its right and left edges with a plurality of pairs of holding points inclined at the angle xcex8 to the running direction of the film, and stretching the film in the xcex8 direction by means of each pair points with running of the film. However, this method also causes crease and wrinkles to be developed in the film, because of different running speed between the right and left edges of the film. For relieving this problem, the stretching stage must be lengthened, which causes installation cost to be greatly increased.
Furthermore, JP-A-2-113920 proposes a method for stretching a film in a direction diagonal to a longitudinal direction of the film, which comprises running the film while holding it between two rows of chucks running on a tenter rail arranged so that the running distances of the chucks within a specified running section on both sides of the film are different from each other. However, this method is also unfavorable for the production of an optical film because crease and wrinkles are developed when the film is stretched in the diagonal direction.
An object of the invention is to provide a diagonal stretching method of a polymer film, which can improve the yield in a stamping process of a polarizer or a birefringencial film.
Another object of the invention is to provide a high-performance, inexpensive polarizer or birefringencial film comprising a diagonally stretched polymer film obtained by the above-mentioned method.
A still another object of the invention is to provide a liquid crystal display device using the above-mentioned polarizer.
As a result of intensive studies for attaining the above-mentioned objects, the present inventors have discovered a method of obtaining diagonal orientation by adjustment of the volatile content in the stretching process and the shrinkage process without development of crease, wrinkles and local unevenness of film thickness.
That is to say, according to the present invention, there are provided a method for stretching a polymer film, a polarizing film, a polarizer, a birefringencial film and a liquid crystal display device each having the following constitution:
1. A method for stretching an optical polymer film by holding both edges of the continuously supplied polymer film by holding means, and imparting tension thereto while advancing the holding means in a longitudinal direction of the film, which comprises allowing a locus L1 of the holding means from a substantial holding initiation point to a substantial holding release point on one edge of the polymer film, a locus L2 of the holding means from a substantial holding initiation point to a substantial holding release point on the other edge of the polymer film, and a distance W between the two substantial holding release points to satisfy the following equation (1), maintaining the supporting property of the polymer film, stretching the film in the presence of a state in which the volatile content is 5% or more, and then, decreasing the volatile content while shrinking the film:
|L2xe2x88x92L1| greater than 0.4Wxe2x80x83xe2x80x83(1); 
(2) The stretching method described in the above (1), wherein L1, L2 and W satisfy the following equation (2):
0.9W less than |L2xe2x88x92L1| less than 1.1Wxe2x80x83xe2x80x83(2); 
(3) The stretching method described in the above (1) or (2), wherein the difference in longitudinal advancing speed between the holding means on both edges of the polymer film is less than 1%;
(4) The stretching method described in any one of the above (1) to (3), wherein an angle made by a center line of a polymer film introduced for holding and a center line of a polymer film sent out to a subsequent step after release of the holding is within 3xc2x0;
(5) The stretching method described in any one of the above (1) to (4), wherein the stretch ratio of the polymer film is from 1.2 to 10;
(6) The stretching method described in any one of the above (1) to (5), wherein the polymer film is stretched in the presence of a state in which volatile content is 7% or more;
(7) The stretching method described in any one of the above (1) to (5), wherein the polymer film is stretched in the presence of a state in which volatile content is 10% or more;
(8) The stretching method described in any one of the above (1) to (5), wherein the polymer film is once stretched at a stretch ratio of 2 to 10 in the presence of a state in which volatile content is 10% or more, and then shrunk 10% or more, thereby inclining an orientation direction of the polymer film at 40 to 50xc2x0 to the longitudinal direction thereof;
(9) A method for stretching a continuously supplied optical polymer film byimparting tension thereto while holding both edges thereof by holding means, which comprises
(i) stretching the film at least in a width direction thereof at a stretch ratio of 1.1 to 20.0,
(ii) adjusting the difference in longitudinal advancing speed between the holding means on both edges of the film to 1% or less,
(iii) bending the advancing direction of the film in the state that both edges thereof are held so that the advancing direction of the film at an outlet of the step of holding both edges of the film is inclined at an angle of 20 to 70xc2x0 to a substantial stretching direction of the film, and
(iv) maintaining the supporting property of the polymer film, stretching the film in the presence of a state in which the volatile content is 5% or more, and then, decreasing the volatile content while shrinking the film;
(10) The stretching method described in any one of the above (1) to (9), wherein the polymer is polyvinyl alcohol, a cellulose acylate, a polycarbonate or a polysulfone;
(11) The stretching method described in any one of the above (1) to (9), wherein the polymer is a vinyl alcohol-based polymer;
(12) A method for producing a polarizing film comprising stretching a vinyl alcohol-basedpolymerby themethod described in any one of the above (1) to (9), and allowing a polarizing element to be adsorbed before or after stretching;
(13) A polarizing film produced by the method of the above (12), wherein the longitudinal direction of the film is inclined at an angle of 20 to 70xc2x0 to a transmission axis direction;
(14) The polarizing film of the above (13), wherein the longitudinal direction of the film is inclined at an angle of 40 to 50xc2x0 to the transmission axis direction;
(15) A polarizer in which at least one side of the polarizing film of the above (13) or (14) is protected with a transparent film;
(16) The polarizer of the above (15), wherein the retardation of the protective film on at least one side at 632.8 nm is 10 nm or less;
(17) Aliquid crystal display device in which thepolarizer of the above (16) is used as at least one of two polarizers disposed on both sides of a liquid crystal cell; and
(18) A birefringencial film produced by stretching according to the method described in any one of the above (1) to (9), in which a longitudinal direction of the film and an orientation direction thereof are inclined at 20 to 70xc2x0 to a parallel.